This invention is directed to a method of separating crude chlorosilane streams containing mixtures of close-boiling chlorosilanes with a distillation aid that is a mono-cyano-substituted organic compound, a nitro-substituted organic compound, a mono-cyano-substituted organosilicon compound, a nitro-substituted organosilicon compound, or a mixture thereof. Lower-boiling chlorosilanes are removed from the crude chlorosilane stream, and the distillation aid and the higher-boiling chlorosilanes are separated from one another.
During the reaction of methyl chloride with silicon, a crude chlorosilane stream is produced. These are normally separated by distillation. Two of the largest volume chlorosilanes are dimethyldichlorosilane Me2SiCl2 and methyltrichlorosilane MeSiCl3. In order to prepare satisfactory siloxane polymers from dimethyldichlorosilane, it is sometimes necessary that the methyltrichlorosilane content of the dimethyldichlorosilane be reduced to trace amounts. The boiling points of methyltrichlorosilane (66.1° C.) and dimethyldichlorosilane (70.1° C.) are sufficiently close that distillation columns of 200 stages or more are required to satisfactorily separate these materials in commercial operations. Consequently, a large capital investment is required in order to install these columns, and it would be highly desirable to reduce this capital investment. Also, a large column generally requires more energy to operate than a smaller column. Careful fractional distillation is also employed by the organosilicon industry to separate other close-boiling chlorosilanes. The present invention is therefore directed to a method of separating crude chlorosilane streams containing close-boiling chlorosilanes such as dimethyldichlorosilane and methyltrichlorosilane which uses smaller distillation columns, and that requires less energy, thereby reducing the capital investment necessary in basic chlorosilane plants.
It is known in the art to use dinitrile compounds which form a separate extract phase that is enriched in difunctional chlorosilane, or dinitrile compounds are used with an additional non-polar wash solvent with low miscibility with the dinitriles, or dinitrile compounds are used as an extraction solvent. By contrast, the present invention does not involve formation of an extract phase with dinitriles, the use of dinitriles with an additional low miscibility non-polar wash solvents, nor does it utilize di-nitrile compounds as an extraction solvent. The present invention uses a mono-cyano-substituted organic compound, a nitro-substituted organic compound, a mono-cyano-substituted organosilicon compound, a nitro-substituted organosilicon compound, or a mixture thereof for separating crude chlorosilane streams containing mixtures of close-boiling chlorosilanes. Mono-cyano-substituted organic compounds are lower boiling materials with normal boiling points that are more than 100° C. lower than the corresponding dinitrile compounds. This distinction allows the separation steps to be conducted at correspondingly lower temperatures, thus avoiding the risk of decomposing the distillation aid in operation.